Democrats contend that marginal tax rates don’t matter to investment and growth, and even some conservative intellectuals are conceding the point. But the evidence from wealth fleeing high-tax states shows how sensitive the affluent are to rate increases.

The liberal tax model is to fleece the rich to finance spending on entitlements and government programs that invariably grow faster than the economy and revenues. IRS data on tax migration show this model is now breaking down in progressive states as the affluent run for...